Method for electrolytic treatment of steel surface in a chromate solution

ABSTRACT

WITH RESPECT TO THE PRIOR ART METHOD WHICH COMPRISES IMMERSING A STEEL PLATE AS A CATHODE IN AN ELECTROLYTE IN WHICH THERE ARE DISSOLVED A MAIN AGENT OF CHROMIC ACID OR CHROMATES AND A PROMOTIVE AGENT OF INORGANIC SULFUR COMPOUNDS, AND SUBJECTING THE SURFACE OF SAID ARTICLE TO ELECTROLYTIC TREATMENT SO AS TO IMPROVE THE PAINT ADHESION AND CORROSION RESISTANCE OF SAID SURFACE, IT HAS BEEN FOUND THAT ADDITION OF FLUORINE COMPOUNDS TO SAID ELECTROLYTE AS A SECOND PROMOTER WILL ELEVATE THE PAINT ADHESION TO THE TREATED SURFACE.

HIDEHISA YAMAGISHI ErAz. 3,679,554 METHOD FOR ELECTROLYTIC TREATMENT OF STEEL SURFACE IN A CHROMATE SOLUTION Filed Jan. 2, 1970 0'5 CONCENTRATION OF- BOROFLUORIC ACID July 25, 1972 United States Patent 3,679,554 METHOD FOR ELECTROLYTIC TREATMENT OF STEEL SURFACE IN A CHROMATE SOLUTION Hidehisa Yamagishi, Yokohama-shi, Hiroshi Takano,

Tokyo, and Masao Takeuchi, Zushi-shi, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Tokyo,

Japan Filed Jan. 2, 1970, Ser. No. 99 Claims priority, applggation Japan, Jan. 13, 1969,

Int. Cl. czsb 11/00 US. Cl. 204-56 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of subjecting the surface of steel plate to electrolytic treatment in a chromate solution so as to coat it with a thin duplex layer having excellent paint adhesion and corrosion resistance.

Heretofore, there have been proposed various methods for subjecting the surface of steel plate, particularly thin steel plate to a so-called electrolytic chromate-treatment to form a duplex layer of metallic chromium and hydrated chromium oxide on the surface thereof, thereby elevating the corrosion resistance, visual appeal and paint adhesion of such articles. All of these methods involve an electrolyte which contains a main agent of chromic acid or chromate dissolved at the concentration of to 100 g. as CrO per liter, namely, at a lower concentration than is used in a simple metallic chromium plating electrolyte, and a promoter, and depositing by electrolytic reduction a duplex layer on the surface of a steel plate immersed as a cathode in the electrolyte. Referring to said promoter, Japanese Pat. No. 269,930 proposes aromatic sulfonic acids or salts thereof, the Japanese Pat. No, 524,872 (US. Pat. 3,484,347) proposes thiocyanic acid or salts thereof and Japanese patent application publication No. 2,768/ 1968 proposes sulfuric acid or salts thereof.

A steel plate whose surface is electrolytically treated with a chromate solution by these prior art methods indeed has an excellent appearance and corrosion resistance, but still are handicapped by their somewhat poor paint adhesion. Where high grade deep drawing is applied on such steel plate after its surface is coated with a paint, part of the coated layer unavoidably comes off.

As a result of studies, applicants have discovered that the aforesaid poor paint adhesion results from the undue thickness of hydrated chromium oxide, forming the upper portion of the duplex layer which is deposited directly on the surface of a steel plate, and that proper reduction of said thickness will noticeably improve the paint adhesion of said surface without affecting its corrosion resistance.

To decrease the thickness of said hydrated chromium oxide, the following three processes may generally be contemplated:

(I) To elevate the temperature of an electrolyte.

(II) To use an electrolyte containing a high concentration of a promoter.

(III) To fully agitate said bath so as to allow it to flow more vigorously toward the cathode.

However, all these processes have their respective drawbacks. Namely, the process (1) leads to decrease the cathode current efliciency for deposited metallic chromium, and to decrease the corrosion resistance of said hydrated chromium oxide layer due to its greater porosity. The process (II) presents difficulties in obtaining a homogeneous and beautiful surface as the concentration of a promoter rises. As far as there is used an electrolytic cell of ordinary construction, the process (III) does not allow the entire cathode surface to be kept in a homogeneous and stable condition simply by mechanical agitation.

There will now be discussed the concentration of a promoter in the electrolyte. This promoter is an indispensable catalyst for electrolytically reducing hexavalent chromium ions in the bath. The present inventors have found that part of said promoter is adsorbed to the layer of hydrated chromium oxide. As electrolysis proceeds, therefore, the concentration of the promoter gradually decreases until the treated surface of a steel plate cannot maintain prescribed appearance and other properties as desired. To assume the stable condition of said treated surface, there must be taken minute care and complicated operation in controlling the concentration of the promoter to a constant level. However, this is a very troublesome and time-consuming process unsuitable for practical work.

An object of the present invention is to coat the surface of steel plate with an extremely thin duplex layer which is excellent in visual appeal, corrosion resistance and paint adhesion.

Another object of the invention is to permit high grade deep drawing for an electrolytically treated steel plate without causing a paint layer coated on its surface to peel off during operation.

The method of the present invention can attain these objects by immersing a steel plate as a cathode in an electrolyte containing chromic acid or chromate as a main agent, inorganic sulfur compounds as a first promotive agent and inorganic fluorine compounds as a second promotive agent, and coating the surface of said steel plate with an extremely thin duplex layer, the upper portion of which consists of hydrated chromium oxide and the lower portion of which consists of metallic chromium.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, as to its organization together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

FIG. 1 is a chart showing the relationship of the various concentrations of fluorine compounds in an electrolyte and the corresponding contents of chromium in the hydrated chromium oxide deposited on the surface of a steel plate; and

FIG. 2. is another chart showing the relationship of the various concentrations of borofluoric acid in the same electrolyte and the corresponding rating of paint adhesion.

An electrolyte used in the method of the present invention contains chromic acid or a chromate dissolved as a main agent, such as chromium trioxide or sodium dichromate consisting of hexavalent chromium. The concentration of said main agent is of the order of 10 to g. CrO per liter. In said electrolyte there is also dissolved a first promotive agent including inorganic sulfur compounds such as thiocyanic acid, thiosulfuric acid, dithionous acid, sulfurous acid, thiosulfurous acid, pyrosulfurous acid, a salt thereof or a sulfide. Said first promotive agent is used at a proportion equal to l/500 to l/25 by weight of chromate ion the agent solution. These two agents play a very important role in connection with the mechanism whereby there is deposited a duplex layer of metallic chromium and hydrated chromium oxide in one step. Electrolysis of an electrolyte containing these two agents results in the partial formation of trivalent chromium ions and further complex salts consisting of said trivalent chromium ion and originally contained hexavalent chromium ion, thereby promoting electrolytic deposition. The technology up to this point is already known (cf. French Pat. 1,168,685).

However, the method of the present invention is characterized in that the electrolyte further contains a second promotive agent including inorganic fluorine compounds, such as hydrofluoric acid, hydrosilicofluoric acid, borofluoric acid, and salts thereof. Said second promotive agent is used at a proportion corresponding to l/500 to l/25 by weight of chromate ion in the solution. It has been disclosed herein that addition of these fluorine compounds enables the hydrated chromium oxide layer to be deposited to a uniformly minute thickness and that adjustment of the concentration of said compounds allows the thickness of said hydrated chromium oxide layer to be controlled to any desired extent, namely the more concentrated said compounds, the thinner will become the hydrated chromiumn oxide layer produced.

Electrolysis according to the method of the present invention is generally conducted under the following conditions:

of electricity Pb, Pb-Sn alloy or Material of anode Pb-Sballoy.

During electrolysis, the concentration of said second promotive agent varies very little. For instance, where electrolysis was conducted by continuously introducing a 4000 111. sheet of stripped steel through 1400 liters of electrolyte, said second promotive agent did not substantially vary in concentration, though the concentration of the first promotive agent changed slightly. However, the effect that the varied concentration of said first promotive agent exerted on the properties of a chromium layer formed was not so prominent as in the case of the prior art binary electrolyte. Accordingly, the method of the present invention permits the concentration of electrolyte components to be controlled with far greater ease than is possible with the prior art.

A thin steel plate whose surface is treated with the chromate solution by the method of the present invention displays excellent formability after painting, and, in the case of can making, fully withstands deep drawing. Further, as the outer portion of the closely deposited layer which consists of hydrated chromium oxide has very excellent corrosion resistance, the surface of the steel plate is free from rusting when the steel plate is left long in the open air until it is painted.

When there is used in can making a steel plate treated with chromate solution according to the present invention, it is preferred that the inner portion of the deposited layer be formed of 0.8 to 1.5 mg./dm. of metallic chromium and the outer portion consists of hydrated chromium oxide containing 0.2 to 0.4 mg./dm. of chromium. Adjustment of the concentration of the second promotive agent enables the deposited layer to be varied in thickness within the aforementioned range according to the application in which said steel plate is to be used.

FIG. 1 gives the relationship of the concentrations of various fluorine compounds dissolved as a second promotive agent in an electrolyte and the chromium content of the hydrated chromium oxide which constitutes the outer portion of the deposit layer. The curves A, B and C of the chart represent the case where there were used in a static state bath sodium silicofluoride Na SiF borofluoric acid HBF, and sodium borofluoride NaBF respectively, as a second promotive agent. The curve D denotes the case where there was added borofluoric acid HBFI, to a flowing state bath. The electrolysis in a static state bath represented by the curves A, B and C was conducted under the following conditions.

Bath composition: 50 g./l. of chromium trioxide, 0.5 g./l. of sodium thiocyanate and 0 to 1.5 g./l. of the various fluorine compounds Current density of cathode: 30 a./dm.

Bath temperature: 50 C.

Treating time: 3 seconds Electrolysis in a flowing state bath associated with the curve D was performed under the same conditions as described above excepting that the current density was changed to 20 arnp./dm. and the treating time was increased to 4 seconds. As apparent from FIG. 1, where the second promotive agent consisted of borofluoric acid, the more concentrated said agent, the thinner became the hydrated chromium oxide which formed to outer portion of the deposit layer. Further, the thickness of said hydrated chromium oxide was generally more reduced when electrolysis was conducted in a flowing state bath than in a static state bath.

As shown in FIG. 2, the thinner said hydrated chro-' mium oxide, the better formability was presented after painting a steel plate treated with a chromate solution. FIG. 2 is a chart showing the relationship of the concentration borofluoric acid corresponding to the curve B of FIG. 1 and the formability after painting of a steel plate treated with chromate solution. The surface of steel plate treated electrolytically in chromate solution was coated with phenol-epoxy resin uniformly at the rate of 50 mg./dm. respectively. Painting of said surface was completed by baking 10 minutes at a temperature of 205 C.

Each sample was subjected to a square can forming test and screw-cap forming test, the rating of paint adhesion obtained from these tests being presented in FIG. 2.

The paint adhesion was evaluated in the following manner. In the square can forming test, there was allotted a maximum of 10 points to the case where the paint did not come off from any of the four corners of the square can. Namely, the greater the paint adhesion, there were allotted larger points. Where there occurred peel-offs starting with those portions which had a small radius of curvature, the rating was decreased by 2 for each such peel-off. Again where the peel-off extended up to a portion corresponding to one-fourth or one-third of the entire length of each sample can as measured from its top, the rating was further de-, creased by 1. Where there even appeared a peel-01f at the bottom of the can, there was allotted a zero point.

Referring to the screw-cap forming test, there was observed the extent to which the paint came off in the threaded portion of the cap. Evaluation was made of these observations in comparison with changes in the paint adhesion of a standard sample by allotting points of from zero to 10 according to a determination of a higher degree of paint adhesion. In FIG. 2, the curve E denotes the degree of paint adhesion as determined by the aforesaid forming test, where there was applied painting on the outside of the can, the curve F the degree of such formability where the inside of the can was painted, the curve G the formability of a screw-cap whose inside was painted as determined by the screwcap forming test and the curve H the formability of a screw-cap whose outside was painted, as determined by said screw-cap forming test.

As apparent from FIGS. 1 and 2, addition of fluorine compounds as a second promotive agent to the prior art electrolyte, containing chromic acid or chromate as a main agent and sulfur compound as a promotive agent, permitted the formation of a thinner coating of hydrated chromium oxide than was possible with the prior art chromate-treating method, and elevated the formability after painting of a steel plate subjected to said novel type of treatment in a so-called chromate solution. It is also shown that a larger addition of said fluorine compounds of the proportion range of from 1/500 to 1/25 by weight of chromate ion in the solution gave better results. Thus the method of the present invention has made it possible freely to produce steel plates adapted for high grade deep drawing by adjusting the concentration of said fluorine compounds as desired.

EXAMPLES Example 1 A cold stripped steel plate 0.23 mm. thick was electrolytically degreased in a solution of sodium silicate, and also electrolytically washed in a dilute sulfuric acid solution and finally subjected to a chromate treatment in an electrolyte having the following composition and under the following treating conditions.

Composition of electrolyte, g./l.:

Chromium trioxide 50 Sodium thiocyanate 0.5 Borofiuoric acid 1.0 Bath temperature, C. 50 Current density of cathode, amp./dm. 20 Treating time, seconds 4 The cathode used in this example consisted of a lead plate whose surface was coated with lead peroxide.

The inner portion of a deposited layer on the treated surface of said steel plate contained 1.0 to 1.1 mg./dm. of metallic chromium, and the outer portion consisted of hydrated chromium oxide contained 0.25 to 0.30 mg./dm. of chromium. This steel plate displayed good paint adhesion for paints containing phenol resin, phenolepoxy resin or phenol-urea resin, respectively.

Other steel plates of the same type as used in the preceding case were subjected to the same treatment as described above excepting that borofiuoric acid was not added. These steel plates and commercially available chromate-treated steel plate were put to the square can forming test, and screw-cap forming test respectively, the comparative results being presented in Table 1 below. The paint coated on the surface of said three types of steel plates for these tests contained a phenol-epoxy resin. It was applied at the rate of 50 mg./dm. and baked 1-0 minutes at a temperature of 205 C. The rating given in Table 1 below represents values averaged through two experiments of each test.

TABLE 1 Comparative rating of paint adhesivity of steel plates Example 2 Tests were conducted in the same manner as in Example 1, excepting that there was used an electrolyte composed as follows:

G./l. Chromium trioxide 50 Sodium thiocyanate 0.5 Hydrosilicofluoric acid 0.5

There were obtained substantially the same results as in Example 1.

Example 3 Example 4 Steel plates of the same type as used in Example 1 were treated with an electrolyte having the following composition and under the following treating conditions.

Composition of electrolyte, g./l.:

Chromium trioxide 50 Sodium thiosulfate (Na S O -5H O) 0.75

Sodium silicofluoride 0.5 Bath temperature, C. 55 Current density of cathode, amp./dm. 40 Treating time, seconds 3 There were obtained substantially the same results as in Example 1.

Example 5 Tests were made in the same manner as in Example 1, excepting that there was used an electrolyte composed as follows:

G./l. Chromium trioxide Sodium bisulfite 1.2

Sodium fluoride v 0.75

There were obtained substantially the same results as in Example 1.

What is claimed is:

1. In a method for electrolytic treatment of a steel surface wherein a steel plate serving as a cathode is immersed in an electrolytic solution containing chromate ions in a concentration of from about 10 to about 100 grams per liter, and wherein the solution is electrolyzed to deposit a thin duplex layer of metallic chromium and hydrated chromium oxide on the surface of said steel plate, the improvement which comprises an electrolytic solution consisting of chromic acid or a chromate, an inorganic sulfur compound selected from the :group consisting of thiocyanic acid, thiosulfuric acid, dithionous acid, sulfurous acid, thiosulfurous acid, pyrosulfurous acid, salts thereof and a sulfide, and an inorganic fluorine compound selected from the group consisting of hydrofluoric acid, hydrosilicofluoric acid, borofluoric acid and salts thereof.

2. The method according to claim 1, wherein an inorganic sulfur compound is present in the electrolytic solution in a proportion by weight equal to 1/500 to 1/25 of chromate ions therein.

3. The method according to claim 1, wherein an inorganic fluorine compound is present in the electrolytic solution in a proportion by weight equal to 1/500 to 1/25 of chromate ions therein.

References Cited UNITED STATES PATENTS 3,518,169 6/1970 Oyama et al. 20456 3,337,431 8/1967 Kitamura et al. 20456 3,484,347 12/1969 Yamagishi et al. 20456 3,257,295 6/ 1966 Yonezaki et al. 2045 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

